The effects of low levels of air pollution and weather conditions on the number of patients admitted to hospitals for exacerbation of chronic bronchitis or emphysema (n = 2807) was studied in Helsinki during a 3-year period, 1987-1989. The daily number of admissions via the emergency room was significantly associated with prevailing levels of sulfur dioxide (SO2) and nitrogen dioxide (NO2) in Poisson regressions controlled for weather, season, time trends, and day of the week, whereas the total number of admissions (via the emergency room and otherwise) was not significantly associated with these pollutants. The effect of SO2 was observed only among those under 65 years old; a significant peak of admissions was seen during the same day (RR, 1.31 for a 2.7-fold increase in SO2; 95% CI, 1.01-1.70; P = 0.039), and another after a 3-day lag (RR, 1.39; 95% CI, 1.05-1.86; P = 0.021). The effect of NO2 was strongest after a 6-day lag and was significant only among those over 64 years old (RR, 1.31; 95% CI, 1.03-1.66; P = 0.022). The average of mean 24-hr concentrations of NO2 was 39 micrograms/m3 (0.021 ppm) and that for SO2 was 19 micrograms/m3 (0.0067 ppm). No relationship was found between admissions and the concentrations of total suspended particulates (TSP) or ozone (O3), the temperature, or the relative humidity. However, the number of admissions among those over 64 years of age was significantly lower, irrespective of temperature, during the summer than during other seasons. The mean daily concentration of O3 was fairly low (22 micrograms/m3 or 0.011 ppm), but that of TSP was high, 76 micrograms/m3. the mean temperature was low, +4.7 degrees C. These results suggest that SO2 and NO2 concentrations lower than those given as guidelines in many countries, and lower than previously shown, may increase the incidence of symptoms in some patients with chronic bronchitis or emphysema.